Degenerative and congenital disease states of the human spine are generally characterized by changes in the anatomic relationship between vertebral segments such that there is a geometric variance from “normal”. Degenerative changes often involve disruption or loss of normal soft tissue associated with the spinal column. Additionally, loss of bone density and changes in absolute geometry of boney spinal elements are often a component of degenerative disease processes that will require surgical treatment. Various fixation devices consisting of rod or plate mechanisms have been applied to the human spine in attempting to correct these deformities and re-approximate diseased anatomy towards “normal”. Most previous designs have rigid structural properties wherein fusion of spinal segments has been the mode of treatment. Additionally, the procedures required for placement of these devices have generally required an open technique with large incisions and commensurate denervation of paraspinus musculature.
Recent developments in spinal instrumentation have emphasized a path towards advancement in percutaneous placement of rod structures, specifically to address the issue of preserving innervation to the paraspinus muscles. Current percutaneous rod systems are generally limited to two or less spinal segments, largely due to the rigid structural property of conventional solid section rods which does not permit deformation for placement nor readily conform to the natural compound curvature of the spine. These systems are almost universally directed towards achieving fusion of the affected spinal segments. The physiologic action of these systems is one of fusion, wherein the device is designed to maximize rigidity between the diseased spinal elements and post-operatively a boney fusion is produced through the normal reparative process and interoperative placement of autologous or artificial bone graft. In essence these systems are directed towards a conventional rigid fixation placed in a percutaneous manner.
Presenting a different approach, several new devices are available that seek to create a physiologic repair that preserves some motion at the instrumented spinal segments; such systems are termed to have properties of “dynamic stabilization”. As advances are made in understanding the pathophysiologic processes involved in degenerative spine disease; particularly those processes involved with chronic inflammatory change, devices and process will be implemented that augment native spinal structure while preserving patient motility. A system that can literally “augment” native structure may alleviate much of the pathology of degenerative spine disease. Furthermore, maintaining some degree of motion may offer the patient advantages over older conventional systems that are rigid in nature. Dynamic stabilization systems potentially offer the following advantages: enhanced patient motility that preserves function and improves the level of patient comfort; motion preservation that may reduce post surgical morbidity, in that continued motion at diseased segments can decrease loss of bone density; a construct that provides dynamic stabilization may provide “load sharing” between diseased segments and those adjacent to it; and variable rigidity from one spinal segment to the next, allowing treatment planning to reflect structural needs of each segment taken individually. These principles of “dynamic stabilization” will likely offer a profound advantage over current rigid fixation devices in that there is a reduced probably of the occurrence of adjacent segment degenerative change; prevention of degenerative changes at adjacent segments will likely significantly reduce the probability of re-operation for the patient and reduce the incidence of continued radicular pain post operatively.
There is a need for a device comprised of multiple relatively small cross-sectional rods that form a controlled rigidity structural construct and permit placement in a percutaneous manner. Structural properties of each rod may be varied along its length with varied material composition or size or shape of cross-section. The rods may be made of materials having “memory” properties such that after placement the rods gravitate towards an idealized shape. Each rod can have relatively flexible structural properties allowing significant deformation of the rod to occur during the placement process.
There is a need for a device that is applicable to various disease states of the human spine. Such a device may act as an “internalized splint” that provides continuous distractive or curvature corrective forces well past the time of surgical placement affecting the global geometry of the diseased spine, or exerts an immediate corrective and stabilizing effect upon the degenerative spine at the time of surgery.
Extensive research of the available instrumentation devices available for treatment of juvenile scoliosis has revealed treatment modalities that are largely limited in degree of correction to that which can be obtained at the time of surgery. Furthermore these types of instrumentation are often applied in a manner that seeks to create multiple fusions. Devices of this type cause significant reduction in patient motility and commensurate increased morbidity due to the nature of rigid fixation. An additional limitation to current devices is that they are typically applied after the point or near the point of bone maturity; this limitation effectively delays the time of appropriate treatment and ignores the potential gains that may be realized through bone remodeling. The maximal corrective change of spinal geometry that can be obtained will be dependent upon the ability to initiate correction prior to skeletal maturity.
Conventional rigid fixation devices likewise are not easily exchanged to accommodate changing size as a juvenile patient grows. The techniques of placement with conventional devices often will involve osteotomies of the vertebral bodies thus precluding use in a patient population with significant remaining skeletal growth. There is a need for a device that addresses the issue of patient growth in juveniles, and which provides for longitudinal expansion as the patient develops.
Scoliosis treatment has employed external bracing systems that have been in use for decades, whereby very effective results have been realized. Effective treatment with external bracing modalities however, comes with the caveat of absolute patient compliance. The reality of treatment with bracing has historically fallen significantly below expected results. The primary issue precluding effective treatment using external bracing has been singularly the lack of patient compliance in an adolescent, image conscious population. There is a need for an internalized surgically placed bracing system that precludes failure to wear the device.
Degenerative processes involving the lumbar spine are certainly the largest segment of the spinal instrumentation market. There is a need for a system that is placed percutaneously and affects dynamic stabilization of the spine while spanning greater than two spinal segments. A “load sharing” construct has certain theoretical advantages over currently available systems: adjacent segment degenerative changes may be attenuated or avoided altogether; distribution of loads may break cycles of chronic inflammation and pain so characteristic of lumbar and thoracic pathologies at multiple levels; and ultimately, treatment planning with controlled degrees of rigidity may be applied to different spinal segments thus yielding a greater degree of motility than can be obtained with present rigid fixation systems with uniform structural properties along their length.